Mast or whip antennas mounted on the exterior of a vehicle such as an automobile have been used for receiving and transmitting electromagnetic signals (e.g., radio waves). It is also possible to place embedded wire antennas of quarter or half wavelength in laminated rear windows of vehicles. Such wires are easily visible and are therefore prohibited for use in the front windshield or side windows of an automobile.
The use of thin-film antennas has been gaining popularity in recent years. Thin-film antennas are generally formed by applying a thin layer of conductive material to sheets of plastic film such as polyester, and then patterning the resulting sheets to form the conductive surfaces of antennas. Alternatively, conductive material may also be deposited on plastic or other dielectric sheets in desired patterns to form the antennas with the use of well-known masking and deposition techniques.
One area where there has been increased interest in using such thin-film antennas is for window-mounted applications in motor vehicles, aircraft, and the like. Due to the increasing need for different modes of wire-less communication, thin-film window antennas represent a desirable alternative to populating a vehicle or aircraft structure with mast antennas, or other types of non-conformal type antennas, which can detract from the aerodynamic and aesthetic appearance of the surface.
Thin film antennas designed to be affixed to any window of the vehicle (e.g., the windshield) are known in the art. An example of such an antenna is described in U.S. Pat. No. 5,083,135 to Nagy et al., entitled “Transparent film antenna for a vehicle window.” Nagy et al. disclose using a transparent conductive film, such as indium-tin-oxide (ITO), for the antenna conductor. However, they acknowledge that there is a compromise between transparency, which requires a very thin layer of ITO, and sufficient film conductivity to give good antenna performance, which requires a thick layer of ITO. Nagy et al. attempt to solve this problem by placing the antenna very high up on the windshield to inductively couple it to the car body. This substantially restricts design freedom.
There is a conflict between the optical transparency and the conductivity (or surface resistance) of thin-films utilized to make such antennas. For example, copper films having a surface resistance of about 0.25 milliohms/square are commercially available, but their transparency is well below the desired level of 70%. Other commercially available thin-films formed from conductive materials such as ITO or silver have acceptable transparencies (for example, AgHT™ silver type films have optical transparencies greater than 75%), but such films have surface resistances in the range of 4-8 ohms/square, which is several orders of magnitude greater than that of the above copper films, or conventional conductors used for antenna construction. When transparent thin-films having a higher surface resistance are used as the conductive surfaces for an antenna, the performance of the antenna is substantially diminished. Antenna efficiency is reduced due to ohmic loss in the higher resistance films, and as a result, antenna gain can be reduced by as much as 3-6 dB, depending upon the type of antenna.
The transparency of window-mounted thin-film antennas is an important consideration, but other factors are important to meet the requirement of invisibility. U.S. Pat. No. 7,656,357 to Ishibashi et al., entitled “Transparent antenna for vehicle and vehicle glass with antenna,” describes a transparent antenna to be installed on a glass surface of a vehicle. Ishibashi et al. achieve good antenna performance and light transmittance using a copper foil with a low-reflection treatment and photoetching to remove copper resulting in a sparse mesh pattern. To further reduce the visibility of the mesh, Ishibashi et al. require a gradation region of progressive wider openings and sparser lines in the mesh pattern at the outlines of each mesh. U.S. Pat. No. 9,231,213 to Song et al., entitled “Methods for integrating and forming optically transparent devices on surfaces,” describes coplanar waveguide antennas where the edges of the mesh are critical to the radiation emitted and need to be the highest conductivity region in the mesh and should not be compromised to reduce visibility.
In the past, attempts have been made to improve the efficiency of transparent thin-film antennas by increasing the conductivity of the surface. This is typically accomplished by increasing the thickness or type of conductive material applied, or by placing relatively thick sheets of non-transparent highly conductive material on the antenna. In doing so, the antennas become non-transparent.
U.S. Pat. No. 8,941,095 to Song et al., entitled “Methods for integrating and forming optically transparent devices on surfaces,” describes an ideal thin-film antenna as having regions of perfect electrical conductivity and regions of glass, which is an insulating dielectric. Song et al. propose using films of silver nanowire or graphene or other conductive materials such as ITO, that all give compromised performance. U.S. Pat. No. 9,504,164 to Ramakrishnan et al., entitled “Manufacturing of high resolution conductive patterns using organometallic ink and banded anilox rolls,” describes a method of making a touch sensor which includes flexographically printing a pattern of thin lines on a region of a substrate, and then plating the pattern to create a pattern of microwires on the region of the substrate that exhibits high conductivity and high transparency. Areas that are not patterned and plated remain in an insulative state. The method described by Ramakrishnan et al. can be adapted to make a thin-film antenna, however the viewing conditions for a touch screen application is very different from that of a transparent antenna.
The touch screens of Ramakrishnan et al. are applied onto a digital display for applications that require touch input, such as cell phones and computers. In these applications, the touch screens have a very uniform pattern throughout the functional area; the touch screens are back lit (typically by LED displays); and the viewing conditions are normally in relatively low intensity indoor lighting environments.
In contrast, window mounted thin-film antennas are typically used in brightly lit viewing conditions (e.g., outdoor sunlight), the antenna pattern is typically much smaller than the entire glass surface to which it is affixed, the antenna can be viewed from both sides, with transmissive and reflective visibility requirements, and the conductivity requirement for the conductive regions is typically much higher than for a capacitive touch screen.
There remains a need for improved thin-film conducting antennas that can be incorporated into transparent surfaces without compromising antenna performance or unnecessarily obstructing the optical view through such surfaces and that are undetectable to a human observer.